This invention relates to monitoring cigarette or cigarette filter rod while it is moving in an axial direction, particularly for detection of an open or otherwise imperfect longitudinal seam in a wrapper for the rod.
U.S. Pat. No. 4685475 discloses a cigarette or cigarette filter rod monitoring device in which a stream of air is directed at the rod while it is moving axially and in which a microphone is used to monitor sound caused by the air stream after it has passed in Coanda effect contact with the rod. In particular a seam defect is said to disrupt the air stream to cause a measurable change in the monitored sound. This prior art device has a channel through which the rod passes, a passage communicating with the channel for directing an air stream tangentially at the rod, and a duct also communicating with the channel and additionally with a microphone. The passage through which the rod passes closely surrounds the rod and this can cause potential contamination problems, particularly due to tobacco dust and particles accumulating in the duct. Also, since the device relies on Coanda effect contact of an air stream with the rod it is particularly sensitive to the position of the rod. Probably in view of this the device is located at the end of the garniture where the rod position is reasonably well defined but even there in practice the minor variations in rod position as it passes through the garniture can cause monitoring difficulties. A further disadvantage of the prior art device is that its location at the end of the garniture tends to impede operators during a rod break-in.